The present invention relates to a metal oxide catalyst for use in the production of acrylic acid by a vapor phase catalytic oxidation of a hydrocarbon having three carbon atoms such as propane and propylene.
In general, acrylic acid is produced through a two-stage oxidation process comprising a step of catalytic reaction of propylene with oxygen to acrolein and a step of catalytic reaction of acrolein with oxygen to acrylic acid.
In recent years, on the other hand, for the reason that there is a difference in price between propane and propylene or the two-stage oxidation of propylene requires complicated steps, the production of acrylic acid using propane as a starting material through a one-stage process has been studied. Many proposals have been made on the catalyst for use in such a process. Representative examples of proposed catalysts include [V, P, Te]-based catalyst [as disclosed in xe2x80x9cCatalysis Todayxe2x80x9d, 13, 679 (1992)], AgBiVMoO (as disclosed in JP-A-2-83348 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d)), BiMo12V5Nb0.5SbKOn (as disclosed in U.S. Pat. No. 5,198,580), [Mo, Te, V, Nb]-based catalyst (as disclosed in JP-A-6-279351), and [Mo, Sb, V, Nb]-based catalyst (as disclosed in JP-A-9-316023 and JP-A-10-137585).
Furthermore, JP-A-10-230164 discloses an improvement in the catalyst for the production of acrylic acid disclosed in the above cited JP-A-9-316023 and JP-A-10-137585, i.e., process for the production of [Mo, Sb, V, Nb]-based oxide catalyst involving a first step of reacting V+5 and Sb+3 at a temperature of 70xc2x0 C. or higher in the presence of Mo+6 in an aqueous medium and bubbling molecular oxygen or a gas containing molecular oxygen through the reaction solution during or after the reaction and a second step of mixing an Nb compound with the reaction mixture and then calcining the mixture.
However, the above proposed catalyst is disadvantageous in that the yield of acrylic acid as the desired product is insufficient or life of the catalyst itself is short. For example, the [Mo, Te, V, Nb]-based catalyst proposed in the above cited JP-A-6-279351 allows the production of acrylic acid in a high yield but is liable to evaporate Te, thereby causing deterioration of catalytic activity with the lapse of time. Further, the [Mo, Sb, V, Nb]-based catalyst disclosed in JP-A-9-316023 and JP-A-10-230164 still has further improvement in the yield of acrylic acid or the reproducibility of production of catalyst. Further, where those catalysts are used for fluidized bed reaction, it is required for the catalysts to exhibit excellent catalytic properties as well as excellent abrasion resistance. Those catalysts are still insufficient in this respect.
As a result of extensive studies to overcome the above-described problems, it has been found that a catalyst which exhibits an excellent abrasion resistance and gives a high yield of acrylic acid can be obtained by adding an aqueous solution of nitric acid or ammonium nitrate together with an Nb compound or a Ta compound at the second step in the process for the production of a metal oxide catalyst for the production of acrylic acid disclosed in the above cited JP-A-10-230164. The present invention has been completed based on this finding.
It is therefore an object of the present invention to provide a metal oxide catalyst which can give a high yield and exhibits an excellent abrasion resistance in the production of acrylic acid by a vapor phase catalytic oxidation of propane.
A first embodiment of the present invention is to provide a metal oxide catalyst comprising Mo, V, Sb, A (A represents Nb or Ta) and optionally other metals for the production of acrylic acid by a vapor phase catalytic reaction of propane, which is prepared through the following steps (1) and (2):
(1) Step of reacting V+5 and Sb+3 at a temperature of 70xc2x0 C. or higher in the presence of Mo+6 in an aqueous medium, and bubbling an oxygen-containing gas into the reaction mixture during or after the reaction; and
(2) Step of adding to the reaction product obtained in step (1) a solution containing a compound comprising A and an aqueous solution of nitric acid or ammonium nitrate, uniformly stirring these components, and then calcining the resulting mixture.
A second embodiment of the present invention is to provide the above-described metal oxide catalyst wherein bubbling the oxygen-containing gas into the reaction solution obtained in step (1) is replaced by addition of hydrogen peroxide to the reaction solution obtained in step (1).